n (context|narrowly) A person who believes that no deity | deities exist (qualifier|especially|one who has no other religious belief).
n (context|broadly) A person who rejects belief that any deities exist (whether or not that person believes that deities do not exist).
n (context|loosely) A person who has no belief in any deities, such as a person who has no concept of deities.
n (context|loosely|uncommon) A person who does not believe in a particular deity (or any deity in a particular pantheon), notwithstanding that they may believe in another deity.